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Abstract:

A gas sensor includes a metal shell, a gas sensor element held in the
metal shell, a metal portion arranged between a flange portion of the gas
sensor element and a seat portion of the metal shell and a porous thermal
spray layer formed on the gas sensor element. When viewed in cross
section along the direction of an axis of the gas sensor, a corner of the
metal portion axially overlaps in position with or radially outwardly
protrudes from an outer surface of a part of the thermal spray layer
located on a side surface of the flange portion and is brought into
contact with the seat portion. A rear end-facing surface of the metal
portion is brought into contact with an outer surface of the thermal
spray layer at a position radially inside the side surface of the flange
portion.

Claims:

1. A gas sensor, comprising: a gas sensor element extending in an axis
direction of the gas sensor and including a bottomed cylindrical element
body and an outer electrode, the element body being exposed to a gas
under measurement at a front end side thereof and opened at a rear end
side thereof and having a radially outwardly protruding flange portion,
the outer electrode being formed on an outer surface of a front end
portion of the element body located front of the flange portion; a
cylindrical metal shell radially outwardly surrounding the gas sensor
element and having, on an inner surface thereof, a seat portion tapered
and inclined toward the front end side so as to face a front end-facing
surface of the flange portion; and an annular metal portion arranged
between the front end-facing surface of the flange portion and the seat
portion, wherein the gas sensor further comprises a porous thermal spray
layer formed on a region from the front end portion of the element body
via the front end-facing surface to at least a side surface of the flange
portion so as to cover the outer electrode; and wherein, when viewed in
cross section along the axis direction, a corner connecting a radially
outer side surface of the metal portion and a front end-facing surface of
the metal portion is formed so as to axially overlap in position with or
radially outwardly protrude from an outer surface of a part of the
thermal spray layer located on the side surface of the flange portion and
is brought into contact with the seat portion, and a rear end-facing
surface of the metal portion is brought into contact with an outer
surface of a part of the thermal spray layer located on the front
end-facing surface of the flange portion at a position radially inside
the side surface of the flange portion.

2. The gas sensor according to claim 1, wherein the side surface of the
flange portion and the front-facing surface of the flange portion are
connected by a curved surface or a plain surface.

3. The gas sensor according to claim 1, wherein, in a region radially
outside the contact of the metal portion and the outer surface of the
thermal stray layer, an angle formed between the rear end-facing surface
of the metal portion and the axis direction is larger than an angle
formed between the outer surface of the thermal spray layer and the axis
direction.

4. The gas sensor according to claim 1, wherein the metal portion is
reduced in thickness toward the radially outside.

5. The gas sensor according to claim 1, wherein the gas sensor further
comprises a protector portion formed integral with the metal portion so
as to extend from the metal portion toward the front end side and
accommodate therein the gas sensor element.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a gas sensor having a gas sensor
element for detecting the concentration of a gas under measurement.

BACKGROUND ART

[0002] As a gas sensor for detecting the concentration of a specific gas
component (such as oxygen or NOx) in an exhaust gas of an automotive
vehicle etc., there is known a gas sensor having a gas sensor element
equipped with a solid electrolyte body. One such type of gas sensor is
known, including a gas sensor element having a cylindrical element body
and an outer electrode formed on an outer surface of the element body and
a metal shell surrounding a radially outer circumference of the gas
sensor element to hold therein the gas sensor element (see e.g. Patent
Document 1).

[0003] As shown in FIG. 12, for example, a gas sensor element 3 has an
element body 3s formed with a radially outwardly protruding flange
portion 3a; and a metal shell 1000 has a tapered seat portion 1000e
formed on an inner surface thereof to hold the gas sensor element 3 by
indirect contact of a front end-facing surface 3af of the flange portion
3a with the seat portion 1000e. An annular plate-shaped metal packing 12
is arranged between the seat portion 1000e of the metal shell 100 and the
flange portion 3a of the gas sensor element 3 (see FIG. 12(a)) and is
deformed along the seat portion 1000e by crimping a rear end portion of
the metal shell 1000, with a filling material 6 filled between the gas
sensor element 3 and the metal shell 1000, and thereby allowing the
filling material 6 to push the flange portion 3a of the gas sensor
element 3 toward the front end side (see FIG. 12(b)).

[0008] In the gas sensor of Patent Document 1, however, an outside
diameter of a corner 12a of the metal packing 12 is smaller than an
outside diameter of the seat portion 1000e of the metal shell 1000 as
shown in FIG. 12(b) so that, when the metal packing 12 is deformed along
the seat portion 1000e of the metal shell 1000, there occurs a radial
displacement of the metal packing 12. Herein, the corner 12j of the metal
packing 12 refers to a corner between a front end-facing surface 12f of
the metal packing 12 and a side surface 12y of the metal packing 12. As a
solution to such a problem, it is conceivable to bring the outside
diameter of the metal packing 12 closer to the outside diameter of the
seat portion 1000e of the metal shell 100 such that the metal packing 12
can be prevented from radial displacement during deformation of the metal
packing 12 along the seat portion 1000e of the metal shell 100.

[0009] By the way, it is effective to cover an outer electrode of the gas
sensor element 3 by a thermal spray coating (thermal spray layer 5) for
the purpose of protecting the outer electrode from a poisoning substance
etc. in a gas under measurement. The thermal spray layer 5 needs to be
formed from a front end portion of the element body 3s to at least a side
surface 3as of the flange portion 3a via the front end-facing surface 3af
of the flange portion 3a (see FIG. 12(b)). This is because of, as an
outer lead (not shown) is formed on the front end-facing surface 3af and
side surface 3as of the flange portion 3a for connection of the outer
electrode to an external circuit, preventing a short circuit between the
lead and the metal shell 1000 and avoiding superimposition of noise onto
the output of the gas sensor.

[0010] By the formation of the thermal spray layer 5 up to the side
surface 3as of the flange portion, there arises a possibility of
separation of the thermal spray layer 5 from the side surface 3as of the
flange portion 3a in the case where the outside diameter of the metal
packing 12 is made closer to the outside diameter of the seat portion
1000e of the metal shell 1000 as mentioned above. The reason for this is
assumed as follows. At the time of placing the metal packing 12 on the
seat portion 1000e of the metal shell 1000, a part of the thermal spray
layer 5 located in the vicinity of a corner 3ac between the side surface
3as and front end-facing surface 3af of the flange portion 3a (more
specifically, a part of the thermal spray layer 5 located radially
outside of the front-facing surface 3af of the flange portion 3a) comes
into contact with the metal packing 12. In such a state, crimping stress
is exerted on the part of the thermal spray layer 5 located on the side
surface 3as of the flange portion 3a in a direction along the side
surface 3as of the flange portion 3a at the time of crimping the rear end
portion of the metal shell 1000. As a result, there occurs a separation
at the interface between the side surface 3as of the flange portion 3a
and the thermal spray layer 5. Although the thermal spray layer 5 is also
formed on the front end-facing surface 3af of the flange portion 3a and
subjected to crimping stress at the time of crimping the rear end portion
of the metal shell 1000, this stress is exerted on the thermal spray
layer 5 in a direction intersecting the front end-facing surface 3af of
the flange portion 3a (as a compression stress) so that there does not
occur a separation at the interface between the front end-facing surface
3af of the flange portion 3a and the thermal spray layer 5.

[0011] There is also known a gas sensor in which a metal shell has a seat
portion for receiving a flange portion of a gas sensor element such that
the seat portion has two step surfaces: a main seat surface and an outer
seat surface extending radially outwardly from the main seat surface to
retain thereon an outer circumferential edge portion of a metal packing
(see e.g. Patent Document 2). This gas sensor is manufactured by placing
the metal packing on the outer seat surface of the metal shell,
accommodating the gas sensor element in the metal shell, and then,
pressing the gas sensor element toward the front end side so as to deform
the metal packing along the main seat surface of the metal shell. The
metal packing can be thus prevented from radial displacement relative to
the step portion of the metal shell. Further, the metal packing has an
inner portion tapered and constricted toward the front end side and
brought into contact with the main seat surface and an outer portion bent
into a horizontal direction from an outer edge of the inner portion.

[0012] Even when a thermal spray layer is formed on the gas sensor element
in the gas sensor of Patent Document 2, there is a gap left between a
part of the thermal spray layer located in the vicinity of a corner of
the flange portion and the outer portion of the metal packing as the seat
portion of the metal shell is formed with a double-step structure and as
the outer portion of the metal packing is bent into the horizontal
direction as mentioned above. The thermal spray layer can be thus
prevented separation from a side surface of the flange portion. However,
it is necessary to adopt a process operation for forming two step
surfaces on the flange portion of the metal shell (i.e. it is necessary
to perform not only forging process but also cutting process etc. for
forming two step surfaces on the flange portion) in the gas sensor of
Patent Document 2. The adoption of such a process operation causes much
expense in time and effort. It is also necessary, at the time of pressing
the gas sensor element toward the front end side by crimping the rear end
portion of the metal shell, to perform the pressing process in two steps
corresponding to the respective two step surfaces of the metal shell. The
manufacturing of the gas sensor becomes time-consuming due to such
pressing process.

[0013] It is therefore an object of the present invention to provide a gas
sensor that has a metal shell, a gas sensor element with a flange portion
and a thermal spray layer and can avoid separation of the thermal spray
layer from a side surface of the flange portion of the gas sensor
element, while preventing a radial displacement of the metal member,
without complicating production of the metal shell.

Means for Solving the Problems

[0014] As a solution to the above problems, there is provided according to
the present invention a gas sensor, comprising: a gas sensor element
extending in an axis direction of the gas sensor and including a bottomed
cylindrical element body and an outer electrode, the element body being
exposed to a gas under measurement at a front end side thereof and opened
at a rear end side thereof and having a radially outwardly protruding
flange portion, the outer electrode being formed on an outer surface of a
front end portion of the element body located front of the flange
portion; a cylindrical metal shell radially outwardly surrounding the gas
sensor element and having, on an inner surface thereof, a seat portion
tapered and inclined toward the front end side so as to face a front
end-facing surface of the flange portion; and an annular metal portion
arranged between the front end-facing surface of the flange portion and
the seat portion, wherein the gas sensor further comprises a porous
thermal spray layer formed on a region from the front end portion of the
element body via the front end-facing surface to at least a side surface
of the flange portion so as to cover the outer electrode; and wherein,
when viewed in cross section along the axis direction, a corner
connecting a radially outer side surface of the metal portion and a front
end-facing surface of the metal portion is formed so as to axially
overlap in position with or radially outwardly protrude from an outer
surface of a part of the thermal spray layer located on the side surface
of the flange portion and is brought into contact with the seat portion,
and a rear end-facing surface of the metal portion is brought into
contact with an outer surface of a part of the thermal spray layer
located on the front end-facing surface of the flange portion at a
position radially inside the side surface of the flange portion.

[0015] In this gas sensor, the corner between the radially outer side
surface and front end-facing surface of the metal portion axially
overlaps in position with or radially outwardly protrudes from the outer
surface of the part of the thermal spray layer located on the side
surface of the flange portion so that the radial space between the metal
portion and the metal shell can be made smaller. This makes it less
likely that the metal portion will be radially displaced in position
relative to the metal shell.

[0016] Further, the seat portion of the metal shell with which the metal
portion is brought into contact is formed to define a single tapered
surface that is inclined (constricted) directly from the inner side
surface of the metal shell toward the front end side. This makes it
possible to facilitate the production of the metal shell without the need
to form a plurality of step surfaces on the inner surface of the metal
shell. This also makes it possible, at the time of pressing the gas
sensor element toward the front end side by crimping the rear end portion
of the metal shell, to eliminate the need to perform a plurality of
pressing steps corresponding to the respective step surfaces of the metal
shell and to press the gas sensor element in one step for reduction of
processing time

[0017] Furthermore, the rear end-facing surface of the metal portion is
brought into contact with the outer surface of the part of the thermal
spray layer located on the front end-facing surface of the flange portion
at a position radially inside the side surface of the flange portion so
that there is a gap left between the part of the thermal spray layer
located in the vicinity of the corner and the metal portion. This makes
it possible to, in the case where an outside diameter of the metal
packing is made closer to an outside diameter of the seat portion of the
metal shell, keep the metal portion from contact with the part of the
thermal spray layer located in the vicinity of the corner and prevent
separation of the thermal spray layer from the side surface of the flange
portion even when crimping stress is exerted on the part of the thermal
stray layer located on the side surface of the flange portion in a
direction along the side surface of the flange portion at the time of
pressing the gas sensor element toward the front end side by crimping the
rear end portion of the metal shell.

[0018] The gas sensor may be configured such that the side surface and
front end-facing surface of the flange portion are connected by a curved
surface or a plain surface.

[0019] The gap can be formed between the part of the thermal spray layer
located in the vicinity of the corner and the metal portion, i.e., the
rear end-facing surface of the metal portion can be brought into contact
with the outer surface of the part of the thermal spray layer located on
the front end-facing surface of the flange portion at the position
radially inside the side surface of the flange portion, only by adjusting
the shape of the corner of the flange shape to a curved surface or a
plane surface.

[0020] The gas sensor may be configured such that, in a region radially
outside the contact of the metal portion and the outer surface of the
thermal stray layer, an angle formed between the rear end-facing surface
of the metal portion and the axis direction is larger than an angle
formed between the outer surface of the thermal spray layer and the axis
direction.

[0021] In this gas sensor, the gap can be formed between the part of the
thermal spray layer located in the vicinity of the corner and the metal
portion, i.e., the rear end-facing surface of the metal portion can be
brought into contact with the outer surface of the part of the thermal
spray layer located on the front end-facing surface of the flange portion
at the position radially inside the side surface of the flange portion
even when the metal portion has a uniform thickness and an ordinary
shape.

[0022] The gas sensor may be configured such that the metal portion is
reduced in thickness toward the radially outside.

[0023] The gap can be formed between the part of the thermal spray layer
located in the vicinity of the corner and the metal portion, i.e., the
rear end-facing surface of the metal portion can be brought into contact
with the outer surface of the part of the thermal spray layer located on
the front end-facing surface of the flange portion at the position
radially inside the side surface of the flange portion, only by reducing
the thickness of the metal portion toward the radially outside.

[0024] The gas sensor may have a protector portion formed integral with
the metal portion so as to extend from the metal portion toward the front
end side and accommodate therein the gas sensor element.

[0025] As the protector portion is formed integral with the metal portion
so as to accommodate therein the gas sensor element, the parts count of
the gas sensor can be reduced for productivity improvement and cost
reduction.

Effects of the Invention

[0026] It is possible according to the present invention to avoid
separation of the thermal spray layer from the side surface of the flange
portion of the gas sensor element, while preventing radial positional
displacement of the metal member, without complicating production of the
metal shell.

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a cross-section view of a gas sensor, taken along the
direction of an axis of the gas sensor, according to a first embodiment
of the present invention.

[0028] FIG. 2 is a perspective view of a gas sensor element formed with an
outer electrode.

[0029] FIG. 3 is a perspective view of a thermal spray layer formed on the
gas sensor element.

[0030] FIG. 4 is a schematic enlarged view of part of FIG. 1, showing a
metal portion in the gas sensor according to the first embodiment of the
present invention.

[0031] FIG. 5 is a schematic view showing a modified example of the metal
portion of FIG. 4.

[0032] FIG. 6 is a schematic view showing another modified example of the
metal portion of FIG. 4.

[0033] FIG. 7 is a schematic view showing still another modified example
of the metal portion of FIG. 4.

[0034] FIG. 8 is a cross-section view of a gas sensor, taken along the
direction of an axis of the gas sensor, according to a second embodiment
of the present invention.

[0035] FIG. 9 is an enlarged view of part of FIG. 8, showing a packing in
the gas sensor according to the second embodiment of the present
invention.

[0036] FIG. 10 is a schematic view showing a modified example of the
packing of FIG. 9.

[0037] FIG. 11 is a schematic view showing another modified example of the
packing of FIG. 9.

[0038] FIG. 12 is a schematic view showing a state where a metal packing
is arranged between a gas sensor element and a metal shell in a
conventional gas sensor.

DETAILED DESCRIPTION OF EMBODIMENTS

[0039] Exemplary embodiments of the present invention will be described
below.

[0040] FIG. 1 is a cross-section view of a gas sensor 100, taken along the
direction of an axis O of the gas sensor 100 (the direction from the
front end to the rear end), according to a first embodiment of the
present invention. In the first embodiment, the gas sensor 100 is an
oxygen sensor for use by insertion into an exhaust pipe of an automotive
vehicle so as to expose a front end portion (protector portion 204 side)
thereof to exhaust gas and detect the concentration of oxygen in the
exhaust gas. Herein, the bottom side of FIG. 1 (protector portion 204
side) is referred to as the front end side of the gas sensor 100; and the
top side of FIG. 1 is referred to as the rear end side of the gas sensor
100.

[0041] The gas sensor 100 includes a metal shell 20 (as a housing) and a
gas sensor element 3 held in the metal shell 20. The gas sensor element 3
is a known type of oxygen gas sensor element having an oxygen
concentration cell structure in which a pair of electrodes is arranged on
an oxygen-ion conducting solid electrolyte body for outputting a
detection value responsive to the amount of oxygen. More specifically,
the gas sensor element 3 has a bottomed cylindrical element body 3s made
of a solid electrolyte and tapered so as to decrease in diameter toward
the front end (see FIG. 2), an inner electrode 3b formed on an inner
circumferential surface of the element body 3s and an outer electrode 3c
formed on an outer circumferential surface of the element body 3s. A
reference gas atmosphere is created in an inner space of the gas sensor
element 3. In this configuration, the gas sensor element 3 detects oxygen
gas upon contact of the gas under measurement with the outer surface of
the gas sensor element 3.

[0042] A radially outwardly protruding flange portion 3a is formed on an
axially middle part of the gas sensor element 3. On the other hand, a
tapered seat portion 20e is formed on a front end part of an inner
circumferential surface of the metal shell 20 so as to decrease in
diameter and become constricted inwardly toward the front end side. An
annular metal portion 202 of a protector 200 is arranged between the
flange portion 3a and the seat portion 20e and is connected to a
protector portion 204 (explained in detail later) of the protector 200.
The gas sensor element 3 is held in the metal shell 20 by inserting the
gas sensor element 3 into the metal shell 20, the flange portion 3a of
the gas sensor 3 into contact with the metal portion 202 of the protector
200 and thereby indirectly brining the flange portion 3a into contact
with the seat portion 20e from the rear end side.

[0043] As shown in FIG. 2, the flange portion 3a of the gas sensor element
3 has a front-facing surface 3af directed toward the front end side of
the gas sensor element 3, a side surface 3as and a corner 3ac connecting
the front end-facing surface 3af and the side surface 3as. In the gas
sensor element 3, the outer electrode 3c is formed on a part of the outer
circumferential surface of the element body 3s located front of the
flange portion 3a. A lead 3cL is formed so as to extend from the outer
electrode 3c toward the rear end side and is connected to an electrode
pad 3cp on a rear end part of the outer circumferential surface of the
element body 3s.

[0044] For the purpose of protecting the outer electrode 3c from a
poisoning substance in the gas under measurement, a porous thermal spray
layer 5 is formed on a region extending from the front end portion 3af
the gas sensor element 3 (element body 3s) to at least the side surface
3as of the flange portion 3a via the front end-facing surface 3af of the
flange portion 3a such that the outer electrode 3c (as a detection
electrode) is covered by the thermal spray layer 5 as shown in FIG. 3. In
order to prevent a short circuit between the lead 3cL, which is located
on the front end-facing surface 3af and side surface 3as of the flange
portion 3a, and the metal shell 20 and to avoid superimposition of noise
onto the sensor output, the thermal spray layer 5 needs to be formed
continuously from the front end portion of the element body to at least
the side surface 3as of the flange portion 3a via the front end-facing
surface 3af of the flange portion 3a. By contrast, at least the electrode
pad 3cp needs to be kept exposed, without being covered by the thermal
spray layer 5, for electrical connection to the after-mentioned outer
terminal 91.

[0045] Referring back to FIG. 1, the metal portion 202 and the protector
portion 204 are formed integrally in the protector 200. The metal portion
202 is tapered and constricted toward the front end side and is brought
into contact with the seat portion 20e. The protector portion 204 is
cylindrical in shape and extends from a front end of the metal portion
202 toward the front end side. The metal portion 202 is engaged between
the flange portion 3a and the seat portion 20e. The protector portion 204
protrudes from a front end portion 20f of the metal shell 20 so as to
accommodate therein a front end portion of the gas sensor element 3,
which also protrudes from the front end portion 20f of the metal shell
20, and has a bottomed cylindrical shape with a front end side thereof
closed and a plurality of holes 204a formed therein to introduce the
exhaust gas to the inside of the protector portion 204.

[0046] The protector 200 can be made of e.g. nickel, nickel alloy or
stainless steel (such as SUS430).

[0047] A cylindrical seal material (talc powder) 6 is filled in a radial
space between the part of the gas sensor element 3 and the metal shell
20, both of which are located rear of the flange portion 3a. A
cylindrical insulating member (ceramic sleeve) 31 is arranged on a rear
end side of the seal material 6. A metal ring (stainless plain washer) 30
is arranged on a rear end side of the insulating member 31. When a crimp
portion 20a is formed by inwardly crimping a rear end portion of the
metal shell 20, the insulating member 31 is pushed toward the front end
side to compress the seal material 6 and thereby fix the insulating
member 31 and the seal material 6 and, at the same time, seal the space
between the gas sensor element 3 and the metal shell 20.

[0048] A radially outwardly protruding polygonal flange portion 20c is
also formed on an axially middle part of the metal shell 20 so as to
engage with e.g. a hexagonal wrench. A male thread portion 20d is formed
on a part of the outer surface of the metal shell 20 between the flange
portion 20c and the front end portion 20f. A gasket (not shown) is fitted
around a step portion between a front end face of the flange portion 20
and a rear end of the male thread portion 20d so as to prevent gas
leakage in the state of mounting the gas sensor to the exhaust pipe. The
front end portion of the gas sensor element 3 is exposed inside the
exhaust gas, by fitting the male thread portion 20d of the metal shell 20
in a screw hole of the exhaust pipe, for detection of the gas under
measurement (exhaust gas).

[0049] A cylindrical metallic outer tube 40 is joined to the rear end
portion of the metal shell 20 so as to cover a rear end portion of the
gas sensor element 3. The outer tube 40 has a front end portion 40a
connected to the metal shell 20, a rear end portion 40b made smaller in
diameter than the front end portion 40a and a step portion 43 located
between the front end portion 40a and the rear end portion 40b.

[0050] A cylindrical insulating separator 111 is arranged inside the front
end portion 40a of the outer tube 40 and is formed with two insertion
holes 115 and 116. Inner and outer terminals 71 and 91 have plate-shaped
base portions 74 and 94 inserted into the insertion holes 115 and 116 and
connection end portions 75 and 95 formed on rear ends of the base
portions 74 and 94 and connected by crimping to leads 141, respectively.

[0051] The separator 111 is thus held in the outer tube 40 by contact of a
rear end face of the separator 111 with the step portion 43 and by
contact of a front end face of the separator 111 with a holder member 45.
The holder member 45 is located front of the separator 111 and fixed in
the outer tube 40 by crimping the front end portion 40a of the outer tube
40.

[0052] The inner terminal 71 also has an insertion portion 73 connected to
the plate-shaped base portion 74 and fitted in a cylindrical hole 3d of
the gas sensor element 3. The insertion portion 73 is cylindrical in
shape and is electrically connected to the inner electrode 3b inside the
gas sensor element 3. The outer terminal 91 also has a cylindrical
portion 93 connected to the plate-shaped base portion 94 and fitted
around the gas sensor element 3. The cylindrical portion 93 is
cylindrical in shape and is electrically connected to the outer electrode
3c (more specifically, electrode pad 3cp) outside the gas sensor element
3.

[0053] A cylindrical grommet 131 is fitted by crimping in the rear end
portion 40b of the outer tube 40 and formed with two insertion holes so
that the leads 141 are led out to the outside from the insertion holes,
respectively. A rear end portion of the grommet 131 is flanged and
enlarged in diameter. The grommet 131 is placed in position by disposing
this enlarged-diameter part on the rear end of the outer tube 40. As the
grommet 131, there can be used a rubber cap made of silicon rubber,
fluoro rubber etc.

[0054] Four circumferentially evenly spaced first air holes 41 are opened
at positions front of the grommet 131 on a lateral surface of the rear
end portion 40b of the outer tube 40 (although only three air holes 41
are shown in FIG. 1). An annular air-permeable filter 50 is fitted
racially around the rear end portion 40b of the outer tube 40 so as to
close the first air holes 41. Further, a cylindrical metallic protection
tube 60 is fitted radially around the filter 50. Four circumferentially
evenly spaced second air holes 61 are opened on a lateral surface of the
protection tube 60 (although only two air holes 61 are shown in FIG. 1).
The outside air can be thus introduced to the inside of the outer tube 40
through the filter 50. The filter 50 is held between the outer tube 40
and the protection tube 60 by crimping the outer tube 40 and the
protection tube 60 at positions front and rear of the first and second
air holes 41 and 61. The filter 50 has a porous structural body made of
resin such as fluoro resin and shows water repellency so as to introduce
a reference gas (outside air) into the inner space of the sensor element
3 without the entry of water from the outside.

[0055] A rear end of the protection tube 60 is bent radially inwardly and
thereby placed on a rear end face of the grommet 131. A through hole 63
is formed in the center of the rear end of the protection tube 60 so that
the leads 141 are led out to the outside from the through hole 63. A rear
end portion of the protection tube 60 is fixed by crimping to the crimped
regions of the rear end portion 40b of the outer tube 40. Namely, the
grommet 131 is arranged in the outer tube 40 by simultaneously crimping
the protection tube 60 and the outer tube 40.

[0056] Next, the characteristic features of the present invention will be
explained below with reference to FIG. 4.

[0057] FIG. 4 is a schematic enlarged view of part of FIG. 1. In the first
embodiment, an outer side surface 202y of the metal portion 202 of the
protector 200 axially overlaps in position with an outer surface 5as of a
part of the thermal spray layer 5 located on the side surface 3as of the
flange portion 3a (that is, the outer side surface 202y of the metal
portion 202 and the outer surface 5as of the thermal spray layer 5 are
radially in the same position). In other words, a corner 202j connecting
the outer side surface 202y and front end-facing surface 202f of the
metal portion 202 axially overlaps in position with the outer surface 5as
of the part of the thermal spray layer 5 located on the side surface 3as
of the flange portion 3a. Herein, an inside diameter d of the seat
portion 20e of the metal shell 20 (i.e. an inside diameter of a part of
the metal shell 20 located rear of the seat portion 20e) is slightly made
larger than an outside diameter of the flange portion 3a so that the gas
sensor element 3 including the flange portion 3a can be prevented from
radial displacement relative to the metal shell 20. The radial space
between the protector 200 and the metal shell 20 is preferably made
smaller (that is, an outside diameter of the metal portion 202 of the
protector 200 is preferably made closer to the inside diameter d of the
seat portion 20e of the metal shell 20) so that, even when the protector
200 is placed on the seat portion 20e, the protector 200 can be prevented
from radial displacement relative to the metal shell 20. For this reason,
it is necessary that the corner 202j between the outer side surface 202y
and front end-facing surface 202f of the metal portion 202 axially
overlaps in position with or radially outwardly protrudes from the outer
surface 5as of the part of the thermal spray layer 5 located on the side
surface 3as of the flange portion 3a.

[0058] Further, the seat portion 20e of the metal shell 20 with which the
metal portion 202 is brought into contact is formed to define a single
tapered surface that is inclined (constricted) directly from the inner
side surface of the metal shell 20 toward the front end side. This
advantageously makes it possible to facilitate the production of the seat
portion 20e of the metal shell 20. This also advantageously makes it
possible, at the time of pressing the gas sensor element 3 toward the
front end side by crimping the rear end portion of the metal shell 20, to
eliminate the need to perform a plurality of pressing steps corresponding
to the respective step surfaces of the metal shell and to press the gas
sensor element 3 in one step for reduction of processing time. As the
seat portion 20e is formed with one tapered surface, the front end-facing
surface 202f of the metal portion 202 is brought into contact with the
seat portion 20e.

[0059] When the seat portion 20e is formed with one tapered surface as
mentioned above, the metal portion 202 is deformed along the seat portion
20e such that a rear end-facing surface 202r of the metal portion 202
becomes flat and extends substantially along a straight line L as viewed
in cross section. Thus, a part of the thermal spray layer 5 located in
the vicinity of the corner 3ac between the front end-facing surface 3af
and side surface 3ac of the flange portion 3a comes into contact with the
metal portion 202 in the case where the front end-facing surface 3af of
the flange portion 3a is formed into a flat shape. There is a possibility
of separation of the thermal spray layer 5 from the side surface 3as of
the flange portion 3a when crimping stress is exerted on the part of the
thermal spray layer 5 located on the side surface 3as of the flange
portion 3a in a direction along the side surface 3as of the flange
portion 3a (i.e. in the direction of the axis O) under the above state.
In view of such a problem, the front end-facing surface 3af and side
surface 3as of the flange portion 3a are connected by a curved surface
(that is, the corner 3ac is rounded off) such that the part of the
thermal spray layer 5 located in the vicinity of the corner 3ac is
axially spaced apart from the metal portion 202 so as to leave a gap G
between the part of the thermal spray layer 5 located in the vicinity of
the corner 3ac and the metal portion 202 as shown in FIG. 4 in the first
embodiment. This makes it possible to keep the metal portion 202 from
contact with the part of the thermal spray layer 5 located in the
vicinity of the corner 3ac and prevent separation of the thermal spray
layer 5 from the side surface 3as of the flange portion 3a even when
crimping stress is exerted on the part of the thermal stray layer 5
located on the side surface 3as of the flange portion 3a in the direction
along the side surface 3as of the flange portion 3a. On the other hand,
the rear end-facing surface 202r of the metal portion 202 is brought into
contact with an outer surface 5af of the part of the thermal spray layer
5 located on the front end-facing surface 3af of the flange portion 3a at
a position radially inside the side surface 3as of the flange portion 3a.
As crimping stress is exerted on the part of the thermal spray layer 5
located on the front end-facing surface 3af of the flange portion 3a in a
direction intersecting the front end-facing surface 3af of the flange
portion 3a (as compression stress along the lamination direction of the
thermal spray layer 5), there does not occur a separation at the
interface between the thermal spray layer 5 and the front end-facing
surface 3af of the flange portion 3a.

[0060] For the purpose of minimizing the stress load exerted on the part
of the thermal spray layer 5 located in the vicinity of the corner 3ac,
it is preferable that the metal portion 202 is shaped in advance to
substantially fit with the seat portion 20e although the metal portion
202 of the protector 200 can be first formed into a flat shape, placed on
the seat portion 203, and then, deformed into a tapered shape by crimping
the rear end portion of the metal shell 20 and pressing the gas sensor
element 3 toward the front end side.

[0061] The element body 3s can be formed into a general shape with the use
of a mold and then finished by grinding with the use of a grindstone. At
this time, it is feasible to round off the corner 3ac by adjusting the
cross-sectional shape of the grindstone for the formation of the corner
3ac.

[0062] FIG. 5 is a schematic view showing a modified example of the
protector 202 of FIG. 4. The configurations of any parts/portions other
than a metal portion 212 of a protector 210 and a flange portion 3a2 of a
gas sensor element 3 in FIG. 5 are the same as those of the gas sensor
100 of the first embodiment. As shown in FIG. 5, the metal portion 212 of
the protector 210 is tapered and constricted toward the front end side
and brought into contact with the seat portion 20e. The protector 210
also has a cylindrical protector portion 214 formed integral with the
metal portion 212 so as to extend from a front end of the metal portion
212 toward the front end side.

[0063] In this example, an outer side surface 212y of the metal portion
212 of the protector 210 axially overlaps in position with an outer
surface 5as2 of a part of the thermal spray layer 5 located on a side
surface 3as2 of the flange portion 3a2 (that is, the outer side surface
212y of the metal portion 212 and the outer surface 5as2 of the thermal
spray layer 5 are radially in the same position). In other words, a
corner 212j connecting the outer side surface 212y and front end-facing
surface 212f of the metal portion 212 axially overlaps in position with
the outer surface 5as2 of the part of the thermal spray layer 5 located
on the side surface 3as2 of the flange portion 3a2. Herein, an inside
diameter d of the seat portion 20e of the metal shell 20 (i.e. an inside
diameter of a part of the metal shell 20 located rear of the seat portion
20e) is slightly made larger than an outside diameter of the flange
portion 3a2 so that the gas sensor element 3 including the flange portion
3a2 can be prevented from radial displacement relative to the metal shell
20. The radial space between the protector 210 and the metal shell 20 is
preferably made smaller (i.e. the outside diameter of the metal portion
212 of the protector 210 is preferably made closer to the inside diameter
d of the seat portion 20e of the metal shell 20) so that, even when the
protector 210 is placed on the seat portion 20e, the protector 210 can be
prevented from radial displacement relative to the metal shell 20. For
this reason, the corner 212j between the outer side surface 212y and
front end-facing surface 212f of the metal portion 202 axially overlaps
in position with the outer surface 5as2 of the part of the thermal spray
layer 5 located on the side surface 3as2 of the flange portion 3a.

[0064] Further, the seat portion 20e of the metal shell 20 with which the
metal portion 212 is brought into contact is formed to define a single
tapered surface that is inclined (constricted) directly from the inner
side surface of the metal shell 20 toward the front end side. This
advantageously makes it possible to facilitate the production of the seat
portion 20e of the metal shell 20. This also advantageously makes it
possible, at the time of pressing the gas sensor element toward the front
end side by crimping the rear end portion of the metal shell 20, to
eliminate the need to perform a plurality of pressing steps corresponding
to the respective step surfaces of the metal shell and to press the gas
sensor element 3 in one step for reduction of processing time. As the
seat portion 20e is formed with one tapered surface, the front end-facing
surface 212f of the metal portion 212 is brought into contact with the
seat portion 20e.

[0065] As shown in FIG. 5, the side surface 3as2 and front end-facing
surface 3af2 of the flange portion 3a2 are not connected by a curved
surface but are directly connected to each other. The front end-facing
surface 3af2 thus extends substantially along a straight line L as viewed
in cross section in FIG. 5. When the metal portion 212 is deformed along
the seat portion 20e such that a rear end-facing surface 212r of the
metal portion 212 becomes flat as in the case of the metal portion 202 of
the first embodiment (see FIG. 4) and extends along the straight line L,
a part of the thermal spray layer 5 located in the vicinity of a corner
3ac2 between the front end-facing surface 3af2 and side surface 3as2 of
the flange portion 3a2 comes into contact with the metal portion 212. In
view of such a problem, the metal portion 212 is reduced in thickness
toward the radially outside such that the part of the thermal spray layer
5 located in the vicinity of the corner 3ac2 is axially spaced apart from
the metal portion 212 so as to leave a gap G between the part of the
thermal spray layer 5 located in the vicinity of the corner 3ac2 and the
metal portion 212. This makes it possible to keep the metal portion 212
from contact with the part of the thermal spray layer 5 located in the
vicinity of the corner 3ac2 and prevent separation of the thermal spray
layer 5 from the side surface 3as2 of the flange portion 3a2 even when
crimping stress is exerted on the part of the thermal stray layer 5
located on the side surface 3as2 of the flange portion 3a2 in a direction
along the side surface 3as2 of the flange portion 3a2. On the other hand,
the rear end-facing surface 212r of the metal portion 212 is brought into
contact with an outer surface 5af2 of the part of the thermal spray layer
5 located on the front end-facing surface 3af2 of the flange portion 3a2
at a position radially inside the side surface 3as2 of the flange portion
3a2. As crimping stress is exerted on the part of the thermal spray layer
5 located on the front end-facing surface 3af2 of the flange portion 3a2
in a direction intersecting the front end-facing surface 3af2 of the
flange portion 3a2, there does not occur a separation at the interface
between the thermal spray layer 5 and the front end-facing surface 3af2
of the flange portion 3a2.

[0066] FIG. 6 is a schematic view showing another modified example of the
protector 200 of FIG. 4. The configurations of any parts/portions other
than a metal portion 212 of a protector 210 in FIG. 6 are the same as
those of the gas sensor 100 of the first embodiment.

[0067] As shown in FIG. 6, the metal portion 212 is reduced in thickness
toward the radially outside as in the case of the protector 210 of FIG.
5. Further, the front end-facing surface 3af and side surface 3as of the
flange portion 3a are connected by a curved surface (that is, the corner
3ac is rounded off) as in the case of FIG. 4. The gap G between the part
of the thermal spray layer 5 located in the vicinity of the corner 3ac
and the metal portion 212 is thus made larger than those of FIGS. 4 and
5. This makes it possible to keep the metal portion 212 from contact with
the part of the thermal spray layer 5 located in the vicinity of the
corner 3ac and assuredly prevent separation of the thermal spray layer 5
from the side surface 3as of the flange portion 3a even when crimping
stress is exerted on the part of the thermal stray layer 5 located on the
side surface 3as of the flange portion 3a in a direction along the side
surface 3as of the flange portion 3a.

[0068] FIG. 7 is a schematic view showing still another modified example
of the protector 200 of FIG. 4. The configurations of any parts/portions
other than a protector 230 in FIG. 7 are the same as those of the gas
sensor 100 of the first embodiment.

[0069] As shown in FIG. 7, the protector 230 has a metal portion 232 and a
protector portion 234 that are the same as those of the protector 200 of
FIG. 4. The protector 230 also has a guide portion 232z formed integral
with the metal portion 232 so as to extend in a cylindrical shape from
the metal portion 232 toward the rear end side. There is a gap G left
between the metal portion 232 and the part of the thermal spray layer 5
located in the vicinity of the corner 3ac as in the case of the protector
200 of FIG. 4.

[0070] In this example, an outside diameter of the guide portion 232z is
made slightly smaller than an inside diameter d of the seat portion 20e
of the metal shell 20. The guide portion 232z can thus fit with the inner
side surface of the metal shell 20 and allow centering alignment of the
protector 230 and the metal shell 20 at the time of placing the protector
230 into the metal shell 20 from the rear end side. In addition, the
protector 230 can be held tightly in the metal shell 20 by arrangement of
the guide portion 232z between the metal shell 20 and the side surface
3as of the flange portion 3a.

[0071] At the time of crimping the rear end portion of the metal shell 20
and pressing the gas sensor element 3 toward the front end side, the
guide portion 232z may be brought into contact with the part of the
thermal spray layer 5 located on the side surface 3as of the flange
portion 3a. In this example, however, stress exerted by the guide portion
232z on the thermal pray layer 5 is sufficiently smaller than crimping
stress exerted on the thermal spray layer 5 in the direction of the axis
O. Under such small stress, there is no possibility of separation of the
thermal spray layer 5 from the side surface 3as of the flange portion 3a.

[0072] Next, a gas sensor 110 according to a second embodiment of the
present invention will be explained below with reference to FIGS. 8 to
10.

[0073] FIG. 8 is a cross-section view of the gas sensor 110 according to
the second embodiment, taken along the direction of an axis O of the gas
sensor 110 and corresponding to the cross section of the gas sensor 100
of FIG. 1. The gas sensor 110 is the same in configuration as the gas
sensor 100 of the first embodiment, except for a packing 300 (as the
claimed metal portion), a front end portion 20f2 of a metal shell 20 and
a protector member 7 formed as a separate part from the packing 300. An
explanation of the same parts and portions of the gas sensor 110 as those
of the gas sensor 100 will be thus omitted herefrom.

[0074] As shown in FIG. 8, the packing 300 is tapered and constricted
toward the front end side and brought into contact with the seat portion
20e. In the second embodiment, the packing 300 is engaged between the
flange portion 3a and the seat portion 20e.

[0075] The protector 7 is made of a metal material (such as stainless
steel) in a cylindrical shape (as a separate part from the packing 300)
and is fixed to the front end portion 20f2 of the metal shell 20 so as to
cover a front end portion of the gas sensor element 3 protruding from the
metal shell 3. Further, the protector member 7 has a plurality of holes
formed therein to introduce the exhaust gas to the inside of the
protector member 7.

[0076] FIG. 9 is a schematic enlarged view of part of FIG. 8. The packing
300 is the same in configuration as the protector 200 of the first
embodiment (see FIG. 4), except for the shape of the packing 3 itself and
the formation of no protector portion 204. Thus, a rear end-facing
surface 300r of the packing 3 becomes flat along a straight line L as in
the case of the protector 200 of the first embodiment. However, there is
a gap G left between the part of the thermal spray layer 5 located in the
vicinity of the corner 3ac and the packing 300 when the side and front
end-facing surfaces 3as and 3af of the flange portion 3a are connected by
a curved surface (that is, the corner 3ac is rounded off). This makes it
possible to keep the packing 300 from contact with the part of the
thermal spray layer 5 located in the vicinity of the corner 3ac and
prevent separation of the thermal spray layer 5 from the side surface 3as
of the flange portion 3a even when crimping stress is exerted on the part
of the thermal stray layer 5 located on the side surface 3as of the
flange portion 3a in a direction along the side surface 3as of the flange
portion 3a. The rear end-facing surface 300r of the packing 300 is
brought into contact with an outer surface 5af of the part of the thermal
spray layer 5 located on the front end-facing surface 3af of the flange
portion 3a at a position radially inside the side surface 3as of the
flange portion 3a. An outer side surface 300y of the packing 300 is
located such that a part of the outer side surface 300y radially
outwardly protrudes from an outer surface 5as of the part of the thermal
spray layer 5 located on the side surface 3as of the flange portion 3a
and another part of the outer side surface 300y remains radially inside
the outer surface 5as of the thermal spray layer 5. Namely, the outer
side surface 300y of the packing 300 overlaps in position with the outer
surface 5as of the part of the thermal spray layer 5 located on the side
surface 3as of the flange portion 3a. Even in this configuration, it is
less likely that the packing 300 will be radially displaced in position
relative to the metal shell 20 when a corner 300j connecting the outer
side surface 300y and front end-facing surface 300f of the packing 300
radially outwardly protrudes from the outer surface 5as of the part of
the thermal spray layer 5 located on the side surface 3as of the flange
portion 3a in the second embodiment. The packing 300 is brought at the
front end-facing surface 300f thereof into contact with the seat portion
20e.

[0077] FIG. 10 is a schematic view showing a packing 310 as a modified
example of the packing 300 of FIG. 9. The packing 310 of FIG. 10 is the
same in configuration as the protector 210 of FIG. 5, except for the
formation of no protector portion 214. Even though the side surface 3as2
and front end-facing surface 3af2 of the flange portion 3a2 are not
connected by a curved surface, the packing 310 is reduced in thickness
toward the radially outside, as in the case of the protector 210 of FIG.
5, so as to leave a gap G between the part of the thermal spray layer 5
located in the vicinity of the corner 3ac2 and the packing 310. This
makes it possible to keep the packing 310 from contact with the part of
the thermal spray layer 5 located in the vicinity of the corner 3ac2 and
prevent separation of the thermal spray layer 5 from the side surface
3as2 of the flange portion 3a2 even when crimping stress is exerted on
the part of the thermal stray layer 5 located on the side surface 3as2 of
the flange portion 3a2 in a direction along the side surface 3as2 of the
flange portion 3a2. On the other hand, a rear end-facing surface 310r of
the packing 310 is brought into contact with an outer surface 5af2 of the
part of the thermal spray layer 5 located on the front end-facing surface
3af2 of the flange portion 3a2 at a position radially inside the side
surface 3as2 of the flange portion 3a2. An outer side surface 310y of the
packing 310 is located such that a part of the outer side surface 310y
radially outwardly protrudes from an outer surface 5as2 of the part of
the thermal spray layer 5 located on the side surface 3as2 of the flange
portion 3a2 and another part of the outer side surface 310y remains
radially inside the outer surface 5as2 of the thermal spray layer 5.
Namely, the outer side surface 310y of the packing 300 overlaps in
position with the outer surface 5as2 of the part of the thermal spray
layer 5 located on the side surface 3as2 of the flange portion 3a2. Even
in this configuration, it is less likely that the packing 310 will be
radially displaced in position relative to the metal shell 20 when a
corner 310j connecting the outer side surface 310y and front end-facing
surface 310f of the packing 310 radially outwardly protrudes from the
outer surface 5as2 of the part of the thermal spray layer 5 located on
the side surface 3as2 of the flange portion 3a2. The packing 310 is
brought at the front end-facing surface 310f thereof into contact with
the seat portion 20e.

[0078] In the second embodiment, it is feasible to increase the gap G by
reducing the thickness of the packing 310 toward the radially outside and
connecting the side surface 3as and front end-facing surface 3af of the
flange portion 3a by a curved surface as in the case of the first
embodiment (the example of FIG. 6). It is also feasible in the second
embodiment to form a guide portion integral with the packing 300 such
that the guide portion extends in a cylindrical shape from the packing
300 toward the rear end side as in the case of the first embodiment (the
example of FIG. 7).

[0079] FIG. 11 is a schematic view showing a packing 320 as another
modified example of the packing 300 of FIG. 9. As shown in FIG. 11, the
packing 320 has a uniform thickness and an ordinary shape such that a
radially outer side surface 320y of the packing 320 is at a right angle
relative to a rear end-facing surface 320r of the packing 320. The flange
portion 3a2 is the same in shape as that of FIG. 5 such that the front
end-facing surface 3af2 of the flange portion 3a2 is flat and extends
substantially along a straight line L as viewed in cross section in FIG.
11. The straight line L (the outer surface 5af2 of the thermal spray
layer) forms an angle θ2 relative to the direction of the axis
O. Herein, an angle formed between the seat portion 20e of the metal
shell and the direction of the axis O in FIG. 11 is made larger than an
angle formed between the seat portion 20e of the metal shell and the
direction of the axis O in FIG. 9.

[0080] On the other hand, a straight line L2 parallel to the rear
end-facing surface 320r of the packing 320 forms an angle θ1
relative to the direction of the axis O. In this example, the angles
θ1 and θ2 are set to satisfy the condition of
θ1>θ2.

[0081] Thus, the part of the thermal spray layer 5 located in the vicinity
of the corner 3ac2 is axially spaced apart from the packing 320 so as to
leave a gap G between the part of the thermal spray layer 5 located in
the vicinity of the corner 3ac2 and the packing 320 even when the rear
end-facing surface 320r of the packing 320 is made flat as in the case of
the metal portion 202 of the first embodiment (see FIG. 4). This makes it
possible to keep the packing 320 from contact with the part of the
thermal spray layer 5 located in the vicinity of the corner 3ac2 and
prevent separation of the thermal spray layer 5 from the side surface
3as2 of the flange portion 3a2 even when crimping stress is exerted on
the part of the thermal stray layer 5 located on the side surface 3as2 of
the flange portion 3a2 in a direction along the side surface 3as2 of the
flange portion 3a2. On the other hand, the rear end-facing surface 320r
of the packing 320 is brought into contact with an outer surface 5af2 of
the part of the thermal spray layer 5 located on the front end-facing
surface 3af2 of the flange portion 3a2 at a position radially inside the
side surface 3as2 of the flange portion 3a2. An outer side surface 320y
of the packing 320 is located such that a part of the outer side surface
320y radially outwardly protrudes from an outer surface 5as2 of the part
of the thermal spray layer 5 located on the side surface 3as2 of the
flange portion 3a2 and another part of the outer side surface 320y
remains radially inside the outer surface 5as2 of the thermal spray layer
5. Namely, the outer side surface 320y of the packing 320 overlaps in
position with the outer surface 5as2 of the part of the thermal spray
layer 5 located on the side surface 3as2 of the flange portion 3a2. Even
in this configuration, it is less likely that the packing 320 will be
radially displaced in position relative to the metal shell 20 when a
corner 320j connecting the outer side surface 320y and front end-facing
surface 320f of the packing 320 radially outwardly protrudes from the
outer surface 5as2 of the part of the thermal spray layer 5 located on
the side surface 3as2 of the flange portion 3a2. The packing 320 is
brought at the front end-facing surface 320f thereof into contact with
the seat portion 20e.

[0082] When the outer surface 5af2 of the thermal spray layer 5 is pressed
against the rear end-facing surface 320r of the packing 320, the rear
end-facing surface 320r is deformed at a contact point P so as to fit
with the outer surface 5af2. In this state, the rear end-facing surface
320r and the outer surface 5af2 are in the same plane at the contact
point P so that the above-defined angles θ1 and θ2
becomes equal to each other (θ1=θ2). The angle
θ1 is thus defined in a region radially outside the contact
point P. The angle θ1 defined in the region radially outside
the contact point P corresponds to the claimed angle formed between the
rear end-facing surface of the metal portion and the axis direction in
the region radially outside the contact of the metal portion and the
outer surface of the thermal stray layer.

[0083] In the example of FIG. 11, the packing 320 is of ordinary shape.
The use of such a packing 320 leads to reduction in parts cost. The
packing 320 is not however limited to the ordinary shape. It is feasible
to reduce the thickness of the packing 320 toward the radially outside as
in the case of the packing 310 of FIG. 10.

[0084] Although the present invention has been described above with
reference to the specific exemplary embodiments, the present invention is
not limited to the above-described exemplary embodiments. Various
modifications and variations of the embodiment described above will occur
within the scope of the present invention.

[0085] For example, the side surface 3as and front end-facing surface 3af
of the flange portion 3a may alternatively be connected by a plain
surface (that is, the corner 3ac may be chamfered at a given angle
relative to the front end-facing surface 3af of the flange portion)
although the side surface 3as and front end-facing surface 3af of the
flange portion 3a are connected by a curved surface (that is, the corner
3ac is rounded off) in the first embodiment as shown in FIG. 4, 6 or 7
and in the second embodiment as shown in FIG. 9.

[0086] Although the outer electrode 3c is formed on the entire part of the
gas sensor element 3 located front of the flange portion 3a as shown in
FIG. 2, the outer electrode 3c may alternatively be formed on some part
of the gas sensor element 3 located front of the flange portion 3a.